Unless otherwise indicated herein, the description provided in this section is not prior art to the claims and is not admitted to be prior art by inclusion in this section.
In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and UEs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to UEs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to UEs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying data from UEs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from UEs to the base station.
In LTE, downlink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines a continuum of 10 millisecond (ms) frames, divided into 1 ms sub-frames and 0.5 ms slots. With this arrangement, each sub-frame is considered to be a transmission time interval (TTI). Thus, each frame has 10 TTIs, and each TTI has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), to form one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a downlink physical resource block (PRB). In the time domain, each downlink PRB has a duration corresponding to one sub-frame (1 ms). In the frequency domain, each downlink PRB consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 1 ms duration of a downlink PRB accommodates 14 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the downlink carrier, the air interface may support transmission on a number of such downlink PRBs in each TTI. For instance, a 5 MHz carrier supports 25 PRBs in each TTI, whereas a 15 MHz carrier supports 75 PRBs in each TTI.
The smallest unit of downlink resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a PRB that consists of 12 sub-carriers and 14 OFDM symbols has 168 resource elements. Further, each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each resource element may represent 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each resource element may represent 4 bits; and with 64QAM, each resource element may represent 6 bits.
Within a PRB, and cooperatively across all of the PRBs of the carrier bandwidth, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements distributed throughout the resource block) may be reserved for reference signals used for channel estimation, and certain resource elements in designated subframes may be reserved to carry synchronization signals or broadcast channels. In addition, a certain number of the resource elements (e.g., resource elements in the first one, two, or three OFDM symbols of each downlink TTI) may be reserved for the PDCCH and other control channels, and most of the remaining resource elements (e.g., most of the resource elements in the remaining OFDM symbols of the TTI) would be left to define the PDSCH.
Across the carrier bandwidth, each downlink TTI of the LTE air interface thus defines a control region or PDCCH region that generally occupies a certain number of 66.7 microsecond symbol time segments (e.g., one, two, or three such symbol time segments), and a PDSCH region that generally occupies the remaining symbol time segments, with certain exceptions for particular resource elements. With this arrangement, in the frequency domain, the PDCCH region and PDSCH region both span the entire carrier bandwidth. In practice, the control region is then treated as being a bandwidth-wide space for carrying control signaling to UEs. Whereas, the PDSCH region is treated as defining discrete 12-subcarrier-wide PDSCH segments corresponding to the PRB definitions across the carrier bandwidth.
One of the main functions of the PDCCH in LTE is to carry “Downlink Control Information” (DCI) messages to served UEs. LTE defines various types or “formats” of DCI messages, to be used for different purposes, such as to indicate how a UE should receive data in the PDSCH of the current TTI, or how the UE should transmit data on the PUSCH in an upcoming TTI. For instance, DCI message in a particular TTI generally schedules downlink communication of bearer data to a particular UE (i.e., a UE-specific data transmission), by specifying one or more PRBs in the current TTI whose usable PDSCH resource elements carry the bearer data, what modulation scheme is used for bearer data in the specified PRB(s), and so forth.
Each DCI message may span a particular set of resource elements on the PDCCH (e.g., one, two, four, or eight control channel elements (CCEs), each including 36 resource elements) and may include a cyclic redundancy check (CRC) that is masked (scrambled) with an identifier (e.g., cell radio network temporary identifier (C-RNTI)) assigned to the UE, so that the UE can identify and read the DCI message. Further, the DCI message may be modulated using QPSK modulation. In practice, a UE may monitor the PDCCH in each TTI in search of a DCI message destined to the UE. In particular, the UE may engage in a “blind decoding” process in which the UE reads various candidate groups of resource elements on the PDCCH in search of a DCI message masked with the UE's identifier. If the UE finds such a DCI message, the UE may then read that DCI message and proceed as indicated. For instance, if the DCI message schedules downlink communication of bearer data to the UE in particular PRBs of the current TTI, the UE may then read the data carried by the PDSCH resource elements of the indicated PRB(s), to receive that bearer data.